The requirements for increasingly high areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), medium noise, i.e., signal-to-noise ratio (SNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the coercivity of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and comprises a substrate 10 of a glass, ceramic or glass-ceramic materials. There are typically sequentially sputter deposited on each side of substrate 10 adhesion enhancement layer 11, 11', e.g., Cr or a Cr alloy, a seedlayer 12, 12', such as nickel-phosphorus (NiP), an underlayer 13, 13', such as Cr or a Cr alloy, a magnetic layer 14, 14', such as a cobalt (Co)-based alloy, and an overcoat 15, 15', such as a carbon-containing overcoat. Typically, although not shown for illustrative convenience, a lubricant topcoat is applied on the protective overcoat 15, 15'.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is recognized that underlayers having a fine grain structure are highly desirable, particular for growing fine grains of hexagonal close packed (HCP) Co alloys deposited thereon.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films which are the underlayer of choice in conventional magnetic recording media. Li-Lien Lee et al., "NiAl Underlayers For CoCrTa Magnetic Thin Films", IEEE Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994. Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer is too low for high density recording, e.g. about 2,000 Oersteds (Oe). The use of an NiAl underlayer is also disclosed by C. A. Ross et al., "The Role Of An NiAl Underlayers In Longitudinal Thin Film Media" and J. Appl. Phys. 81(a), P.4369, 1997.
Conventional practices in manufacturing magnetic recording media comprise Direct Current (DC) magnetron sputtering and high temperatures in order to obtain Cr segregation in Co-alloy grain boundaries to achieve high Hr and high SNR. However, low temperature DC magnetron sputtering techniques can only produce low Hr and low SNR media with NiAl seedlayers and Cr-alloy underlayers.
The demands for increasingly high areal recording density create a need for magnetic recording media exhibiting high Hr and high SNR, particularly in media containing an NiAl seedlayer and a Cr-alloy underlayer.